I. Field of the Invention
This invention relates to cathodes used in electrolysis cells, particularly in the cells used for the production of aluminum metal. More particularly, the invention relates to multi-layer cathode structures used in reduction cells of this type.
II. Description of the Prior Art
In metal reduction cells it is usual to line a container with a carbonaceous material, such as anthracite and/or graphite, and to use the carbonaceous layer as a cathode for the cell. A molten electrolyte is held within the container and carbon anodes dip into the molten electrolyte from above. As electrolysis proceeds, molten metal forms a pool above the cathode layer.
The cathode layer, which normally extends along the bottom wall of the cell and possibly up the side walls to a level above the height of the surface of the molten electrolyte, eventually breaks down and the cell has to be taken out of operation for cathode repair or replacement. This is because the surface and joints of the carbonaceous material are attacked and eroded by the molten metal and electrolyte. The erosion/corrosion of the bottom blocks is a particular problem because of movements of the cell contents caused by magneto-hydrodynamic effects (MHD).
Attempts have been made to make cell cathodes more durable by providing the carbonaceous material with a protective lining. The lining must, of course, be electrically-conductive and, to facilitate the operation of self-draining cathode cells, should be wettable by the molten metal.
Lining materials used for this purpose have included refractory composites made of a carbonaceous component and a refractory metal oxide or boride. Because of its desirable erosion resistance and metal wettability, titanium boride (TiB.sub.2) is a particularly preferred material for use in such composites, despite its extremely high cost. However, the use of this material causes a problem in that it has a different coefficient of thermal expansion compared to that of carbon. During operation at high temperature in the cell, cracks tend to form at the interface of the coating and the underlying cathode carbon, leading to eventual failure of the cathode structure. Thus, the effective service life of the cell is not prolonged as much as would be desired using multi-layer cathode structures of this kind. In fact, although various kinds of cathode structures have been proposed in the past, usually requiring ceramic tiles of TiB.sub.2 adhered to carbon blocks, no such structures are in common use today because the tiles eventually dislodge or crack due to the difference in thermal expansion properties. The same is also true of other composite coating materials, e.g. those containing refractory metals oxides (such as TiO.sub.2 and SiO.sub.2), silicon, nitrides, etc.
A possible solution to this problem would be to provide cathodes structures made entirely of blocks of the composite materials. However, the high cost of such composites (particularly those based on TiB.sub.2), has prevented this as a widespread solution.
An attempt to improve the adhesion of the layers is disclosed in U.S. Pat. No.5,527,442 to Sekhar et al., issued on Jun. 18, 1996. This patent relates to the coating of refractory materials (including titanium borides) onto substrates made of different materials, particularly carbonaceous materials, for use in aluminum reduction cells. To avoid adhesion problems, the coating material is applied as a micropyretic slurry to the carbonaceous substrate which, when dried, is ignited to produce condensed matter forming a coating adherent to the surface of the substrate and thus protecting it. However, such a process is expensive, has not been adopted on a significant industrial scale and also this material has a short operational life.
There is, therefore, a need for an improved way of forming multi-layer cathodes that are not subject to unacceptable rates of dislodgement or cracking of the protective layers.